In curtain coating, a moving receiving surface is coated by the impingement of a free-falling curtain of liquid coating composition. The curtain may be a single coating composition or a composite of several layers of distinct coating compositions, such as described in U.S. Pat. No. 3,508,947 (Hughes), commonly assigned. The coating-receiving surface can be any surface that can be passed through a curtain. For example, the receiving surface can be the surface of a discrete object but is typically the surface of a continuous web of paper, plastic, metal, or cloth.
Various means are well known for forming a free falling liquid curtain. Dies or weirs may be used, for example. A curtain that is a composite of several layers of distinct coating compositions can be formed by a slide die with multiple elements for distributing coating compositions as layers and with an inclined slide surface on which the layers flow by gravity.
Referring to FIG. 1, there is shown generally a curtain coating system comprising two slots 2,3 from which two layers flow superimposed on an inclined flow surface 4. The system includes a lip area/region 5 from which the coating composition leaves the system along a front surface 6, forming a curtain. The curtain then impinges onto a surface, for example a moving support.
Generally, the coating composition falls from a horizontal lip to the coating-receiving surface over a vertical distance between about 2 cm and about 30 cm. The coating composition in the curtain is freely falling and accelerates by gravity.
It is known to employ various edge guide means for maintaining the width of the free falling curtain against surface tension. In some arrangements, each edge guide comprises parallel dual wires, flushing means near the lip of the slide die, and suction means near the coating-receiving surface, as disclosed in U.S. Pat. No. 5,328,726 (Reiter), U.S. Pat. No. 5,725,910 (Devine), U.S. Pat. No. 5,763,013 (Devine), or U.S. Pat. No. 5,976,251 (Devine), all commonly assigned.
There exists a wetting line on the lip where the curtain forms. For a slide die with multiple elements, a front surface of the lip is part of the slide surface over which the composite layer of coating composition flows. There is a wetting line on a partially wetted back surface of the lip. The front surface of the lip may be substantially vertical, and the back surface of the lip may form a sharp angle, such as 30°, with the front surface of the lip. The back surface of the lip is typically upwardly inclined to direct flow into the curtain. The tip of the lip may be sharp or chamfered. An example of a lip is disclosed in U.S. Pat. No. 5,462,598 (Servant), commonly assigned, wherein the wetting line is not positioned at the tip of the lip, but rather at some small distance, less than about 1 mm, along the back surface of the lip.
Ideally, the wetting line is straight and horizontal, but this is typically not the outcome of starting flow and forming a curtain. More likely, the position of the wetting line varies along the lip and may be jagged in some areas. It is known that an irregular wetting line can cause objectionable non-uniformities in the coated layers of highly sensitive products. These non-uniformities in the coated layers usually continue in the coating direction substantially unchanged and are typically referred to as “streaks”. An irregular wetting line can also give rise to stationary waves in the curtain which can also result in streaks in the coated layers.
One known apparatus intended to improve/correct the shape of the wetting line is disclosed in U.S. Pat. No. 5,725,666 (Baumlin), commonly assigned, and is shown in FIG. 2. Baumlin teaches an apparatus for forcing the liquid composition which flows on the forward face of the lip to wet the rear face of the lip of the coating device over a predetermined distance greater than the distance over which the liquid composition would naturally wet the rear face. While this apparatus has achieved a certain degree of success, the apparatus directly contacts the lip.
Another known apparatus intended to correct wetting line shape is disclosed in U.S. Pat. No. 6,346,299 (Gruszczynski), commonly assigned, and is shown in FIG. 3. Gruszczynski employs a movable trough positioned in proximity to the lip. The coating composition flowing on the slide is intercepted by the trough and fills the trough. The level of coating composition wets the back surface of the lip beyond the natural position of the wetting line. The ends of the trough are preferably open to prevent or limit the overflowing of the edges of the trough. The movable trough is then extracted. This apparatus avoids contact with the lip, however, a translation or rotary motion of the trough is required.
When a coating composition is initially supplied to a curtain formation apparatus, splashing and contamination can result if the height of the lip is above the coating zone. A preparation pan with edge walls to minimize splashing and splattering is disclosed in U.S. Pat. No. 5,338,359 (Conroy), commonly assigned, and is shown in FIG. 4. The preparation pan includes flushed edge walls spaced from edge guides to stabilize partial curtains outside the preparation pan. While this apparatus has achieved a certain degree of success, the pan is intricate, and its use alone does not ensure a straight wetting line on the lip. Additional remedies are referenced in Baumlin or Gruszczynski.
Accordingly, there exists a need for an apparatus and method for generating a straight wetting line on the back surface of a lip of a slide die for curtain coating. It is preferred that the apparatus is stationary, does not require physical contact with the lip, and does not result in spillage, splashing, or contamination.